Time-variant multi-path conditions provide a difficult challenge for WCDMA receiver algorithms. In order to achieve good performance, the receiver must know the instantaneous channel conditions or derived figures (depending on the kind of equalizer/combiner) precisely. However, time-variability of the complex channel gains prevents long-term averaging of measurements or estimates of the channel conditions.
Some receivers use a so-called non-parametric GRAKE approach, calculating pilot sequence variation on each RAKE finger for impairment and noise estimation. This approach is explained, for example, in U.S. Pat. No. 6,363,104 B1, issued Mar. 26, 2002 to G. E. Bottomley, and in G. E. Bottomley, T. Ottosson, and Y.- P. E. Wang, “A generalized RAKE receiver for interference suppression,” IEEE J. Sel. Areas in Commun., vol. 18, pp. 1536-1545, August 2000. With this approach, since a single slot does not provide sufficient impairment information to construct an accurate impairment matrix, multi-slot filtering of the impairments is generally employed. However, in faster-changing mobility scenarios, the multi-path environment changes so quickly that impairment matrix filtering actually degrades the performance of the impairment estimation in these receivers. Therefore, this approach is best limited to lower speeds, e.g., up to 10 km/h, depending on the multi-path profile and signal strength.
In another approach, e.g., the convolutional parametric GRAKE approach, impairment matrices are computed directly, using convolution of the received net channel responses. This approach is discussed in co-pending U.S. patent application Ser. No. 12/052,042, titled “Interpolating G-Rake With Two Step Delay Hypothesis Testing” and filed 20 Mar. 2008, the entire contents of which are incorporated by reference herein. This method is capable of providing more accurate channel estimation in faster changing circumstances, thus improving mobility performance compared to non-parametric GRAKE method. However, this approach also suffers performance degradation when filtering of channel estimates is not possible, e.g., at very high speeds. This problem becomes even more significant as more paths are present in the multi-path profile, with each additional path contributing more inter-symbol interference (ISI) to the fingers. Consequently, channel estimation becomes less accurate in these situations.